1. Field of the Invention
This invention relates to FM demodulators, i.e. demodulators or discriminators for demodulating frequency-modulated (FM) signals.
2. Description of the Prior Art
A known type of demodulator or discriminator for an FSK (frequency shift keyed) FM signal, described in UK Patent Specification No. GB 1 517 121, comprises two mixers each connected to receive an FSK input signal, a local oscillator operative to apply an output at the input signal frequency directly to one mixer, a 90.degree. phase shifter operative to apply the local oscillator output to the other mixer with a phase shift of 90.degree., a respective low pass filter connected to filter the output from each of the mixers, a respective limiting amplifier operative to produce a square wave from the output of each filter, and a circuit for sampling one square wave every time the other square wave change amplitude in one sense--i.e. the one square wave is sampled once per period thereof--to reproduce the modulation of the input signal.
The performance of this type of discriminator or demodulator is optimised when the local oscillator frequency is half-way between the two extreme frequencies of the transmitted FM signal. In the case of an FSK signal, these extreme frequencies will be the actual two frequencies being alternately transmitted. It will therefore be apparent that, in the types of demodulator described in UK Patent Specification No. GB 1 517 121, great care must be taken in the design of the local oscillator to ensure an accurate and stable oscillating frequency. This is difficult to achieve in practice, particularly if the circuit is to be used over a wide range of temperatures.
Similar considerations apply to other receiver systems, for example those based on the superheterodyne principle, the local oscillator frequency then being required to be maintained at the centre transmission frequency.